The thesis is devoted to the study of photometrical variability of SU UMa type cataclysmic variables.
The introduction provides a justification for the choice of research topic and its relevance, defines the purpose and objectives of the dissertation, provides methods for solving problems. The connection of work with scientific programs and topics is indicated, details of approbation of results are specified and publications on the topic of the dissertation are given. The scientific novelty of the work is substantiated, as well as the practical significance of the obtained results.
The first section "Current state of research of cataclysmic variables of SU UMa type" contains a review of the literature with the classification of cataclysmic variables, the dependence of their types on the rate of mass loss and the presence of a magnetic field around the white dwarf. The characteristic of light curves for SU Uma type dwarf novae – the main object of research in the dissertation – are given and the main parameters are considered – the duration of the supercycle, the interval between normal outbursts, the duration of outbursts and superoutbursts, their amplitudes.
Modern models are briefly described to explain the outbreak activity of cataclysmic variables. Currently, the most commonly used and generally accepted model is the thermo-tidal instability model. According to this model, the cause of conventional flashes is thermal instability in the accretion disk, while superflashes are caused by the presence of tidal instability, which occurs due to the 3: 1 resonance between the speed of matter in the disk and the speed of orbital movement of the secondary component. An important fact is that achieving such a resonance is possible only for systems with a mass ratio q < 0.3 (q = M2/M1, where M1 is the mass of the white dwarf, M2 is the mass of the donor star), which are cataclysmic variables with orbital periods from 76 up to 191 minutes.
The second section "Features of photometric observations of variable stars: equipment, methods of observation, data processing" lists the tools on which the main observational material was obtained and the applications and programs compiled and developed by the applicant. The section also provides information on the method of obtaining observational material and its primary processing. The study of the photometric system, which consists of the telescope itself, the CCD of the receiver and a certain set of filters is also paid attention in the second section. The construction of a photometric system on the example of the AZT-8 telescope with a ST-8XMEI camera and BVR filters is presented. Vitaly Goransky's WINEFK (http://vgoranskii.net/software) and Jaan Pelt's ISDA (ISDAM) programs were used for the periodogram analysis.
The third section "Results of photometric studies of selected cataclysmic variables of SU UMa type" presents preliminary information and results of studies the cataclysmic variables V1006 Cyg, NY Ser, ASASSN-18fk, Gaia18aak, OV Dra, ASASSN-14ca, ASASSN-15sc. And two of them, namely V1006 Cyg, NY Ser, belong to the objects that are in the "pit of periods".
Examples of V1006 Cyg and NY Ser show that in the case of long-period systems near the tidal instability limit, such solutions at the limit of large q values can occur and lead to significant substance residues in the disks even after flashes. If the rate of transfer of the substance from the secondary component is insufficient, this condition allows to include the phenomenon of vision such as WZ Sgt (multiple brightenings) as in the case of V1006 Cyg. At high transfer rates close to thermal stability, NY Ser-like systems, on the other hand, will mimic the behavior of Z Cam-type stars by showing quasi-constant states produced by large amounts of matter in the disk after a flash combined with high transfer rates.
ASASSN-18fk is the first intermediate polar of a rare class of intermediate polars showing the humps for which the presence of an orbital period tracked during the main flash, repeated brightening, and up to an almost calm state.
The cataclysmic variable Gaia18aak classified for the first time as a dwarf new type of SU UMa and the period of positive sublevels 0.d0674 was determined.
